tests/test-pull-pull-corruption2.t

filectx: fix cmp() of file starting with '\1\n'

If file data starts with '\1\n', it will be escaped in the revlog to
create an empty metadata block, thus adding four bytes to the size in
the revlog size index. There's no way to detect that this has happened
in filelog.size() faster than decompressing each revision [1].

For filectx.cmp(), we have the size of the file in the working directory
available. If it differs by exactly four bytes, it may be this case, so
do a full comparison.

 [1]: http://markmail.org/message/5akdbmmqx7vq2fsg

Corrupt an hg repo with two pulls.
create one repo with a long history

  $ hg init source1
  $ cd source1
  $ touch foo
  $ hg add foo
  $ for i in 1 2 3 4 5 6 7 8 9 10; do
  >     echo $i >> foo
  >     hg ci -m $i
  > done
  $ cd ..

create a third repo to pull both other repos into it

  $ hg init version2
  $ hg -R version2 pull source1 &
  $ sleep 1
  pulling from source1
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 10 changesets with 10 changes to 1 files
  (run 'hg update' to get a working copy)
  $ hg clone --pull -U version2 corrupted
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 10 changesets with 10 changes to 1 files
  $ wait
  $ hg -R corrupted verify
  checking changesets
  checking manifests
  crosschecking files in changesets and manifests
  checking files
  1 files, 10 changesets, 10 total revisions
  $ hg -R version2 verify
  checking changesets
  checking manifests
  crosschecking files in changesets and manifests
  checking files
  1 files, 10 changesets, 10 total revisions